Research On Methane Detection Technique Based On Vehicle Mounted Open-path TDLAS

Methane is the main component of natural gas,marsh gas,oil field gas and coal mine gas,it is a colorless and odorless gas with flammability,and it is in danger of explosion when the amount of methane in the air reach a certain level.With the increasing proportion of natural gas in energy consumption of our country,gas leakage is unavoidable due to the pipeline corrosion cracking and valve tightness in the process of production,processing,transportation and storage of natural gas.Therefore,it is of great significance to ensure the safety of natural gas pipeline transportation and industrial production through methane detection technology with low detection limit and high accuracy to find out the leak point in time and quickly.Tunable diode laser absorption spectroscopy(TDLAS)is a gas concentration detection method.Based on the Beer-Lambert law,TDLAS measure the gas concentration by detecting the change of light intensity through the gas cell.TDLAS has been a widely used spectral detection method in the field of gas concentration detection because of its advantages of high selectivity,high spectral resolution,fast response time and no contact measurement and so on.However,the system noises especially optical interference fringes in the TDLAS system make the background signal fluctuate greatly,and the low concentration gas can not be detected accurately,the false detection or even missed detection to detected gas are caused.In order to suppress the fluctuation of background noise and reduce the system detection limit,the methods of interference fringe noise suppression in the TDLAS detection system are studied to reduce the influence to detection signal in this paper.In addition,the methane detection technique based on vehicle mounted open-path TDLAS is mainly studied,it can expand the detection range and improve the detection efficiency of gas leakage of natural gas pipeline,find out the leakage point accurately and quickly and ensure the safety of natural gas pipeline transportation and industrial production.The main research contents are as follows.Firstly,we design and set up a vehicle-mounted TDLAS methane detection system with open path,based on TDLAS gas detection principle(including the selective absorption mechanism of gas molecules and Beer-Lambert law),wavelength modulation spectroscopy and harmonic detection techniques.The research focus on the selection of optical components such as laser and photoelectric detector,the design of open gas absorption cell,the design of unit circuits including digital lock-in amplifier and function generator et.al.,and the design of the anti-vibration structure of the vehicle-mounted detection system.Secondly,we analyze the source and formation mechanism of optical interference fringes,and study the theoretical model and influence on the detection signals of interference fringes.For enhancing the signal-to-noise ratio of detection signals and reducing interference fringe’s influence to background signal,we apply the dual-tone modulation technique to modulate the laser using two high frequency current signals.The effectiveness and the stability of the dual-tone modulation technique on reducing the background fluctuation are tested by simulation and experiment of gas concentration detection and background signals acquisition in TDLAS system.Thirdly,we study the reflector vibration technique in optical path to change the optical length and reduce the background fluctuation.In the TDLAS detection system,a woofer is used to realize the vibration of the reflector.The effect of optical path length modulation on etalon transmission function,second harmonic background signal and detection signal are analyzed theoretically and verified in TDLAS experimental detection system.The vibration reflector is combined with the dual-tone modulation technique and applied to TDLAS experiment system and vehicle-mounted system.The performances of the combination on reducing the background fluctuation and detection limits are further tested by analizing the background signals and gas concentration detection signals.Finally,in order to suppress the influence on detection signal of interference fringes generated inside the optical elements of TDLAS system,we study the detection signals’ post-processing method based on empirical mode decomposition.The characteristics of TDLAS detection signal are analyzed,the application mechanism of empirical mode decomposition in TDLAS system is introduced,and the suppression principle of empirical mode decomposition method to the background signal fluctuation is given.After the empirical mode decomposition and correlation analysis of the detection signals,the signal components contributing greatly to the background fluctuation are selected and eliminated.We process and analyze the background signals,second harmonic signals and the gas concentration detection signals which are collected in the experiments carried out on laboratory testing platform and vehicle-mounted system.The results indicate that the detection signals’ post-processing method based on empirical mode decomposition and correlation analysis is effective in reducing the background fluctuation and improving the system detection accuracy.